"Asymmetric polarization" is the term political scientists use when one party becomes way more radical than the other. Most recently, it's the term Thomas Mann and Norm Ornstein have used for the Republican Party becoming way more radical than the Democratic Party.
Political scientists tend to back up their case with graphs like this one, which use standard poli-sci measures of polarization that rely on long-term analysis of coalitions and...you've stopped listening, haven't you?
In her profile of House Speaker John Boehner, Jennifer Senior relays the kind of anecdote that anyone who reports on politics already knows but whose importance in this debate is really underrated:
The irony is Boehner was once one of those upstarts himself. The 1994 Gingrich revolution is what made his career take off, just as the 2010 tea-party rebellion made Michele Bachmann’s career take off, and everyone else’s who today causes Boehner headaches. Back in 1994, seasoned Republicans like Bob Dole—himself once a hothead in his political youth—could not get over how radicalized Boehner’s young generation had become.
Today, Boehner is Bob Dole: a befuddled party leader who can’t believe all these crazy kids in his midst, these shameless ideologues who have a total-victory policy and can’t be content with getting 90 percent of the loaf (which was Dole’s famous standard for any worthy piece of legislation).
It's common for one-time Republican revolutionaries to lament -- usually, though not always, off-the-record -- the rowdiness and extremism of their newer colleagues. In 1996, they were that Ronald Reagan and George H. W. Bush knew how to raise taxes when they needed to, and understood that government could do good as well as bad. In 2013, they're complaining that at least Newt Gingrich knew that at the end of the day, you had to get to a deal, and you couldn't really breach the debt ceiling, and you couldn't let your members run all over you.
Every political reporter has heard this kind of thing. But they don't hear it -- at least not in the same way -- on the left. Democrats from 1988 weren't shocked by the liberalism of Democrats in 1996. If anything, they were shocked by their conservatism. The Democratic coalition split that year over welfare reform.
And Democrats from 1996 aren't scratching their heads over the unreasonableness of Democrats in 2013. Again, if they do criticize the current administration, it tends to be in lamenting the moderation and spinelessness of President Obama's White House. Clintonites are sure that their boss would've stared Republicans down when they threatened to breach the debt ceiling. Harder-core liberals can't believe Obama embraced the individual mandate that Republicans came up with and Democrats once opposed as a sop to private insurers.
That older Republicans tend to criticize their colleagues for moving right and older Democrats also tend to criticize their colleagues for moving right is, by the way, a value-neutral fact. Perhaps you think the problem with Republicans in 1986 was that they were too liberal. Perhaps you think Democrats have shifted far to the right. There's no reason to believe that the positions of the two parties in 1993 was somehow optimal.
Still, simply as a piece of evidence in the "is-asymmetrical-polarization-real" debate, if you just listen to multiple generations of the two parties complain, the fact of asymmetric polarization presents itself pretty clearly.